Conventionally, projection display apparatuses using various types of spatial light modulators have been known as video equipment for use with a large screen. Recently, reflective spatial light modulators with high display efficiency, such as a DMD (Digital Micro-Mirror Device), have been receiving attention (see JP 2000-98272A, for example).
FIG. 29 shows a configuration of a projection display apparatus using a DMD as a spatial light modulator. FIG. 29A shows a plan view, and FIG. 29B shows a side view. Moreover, FIG. 30 shows a schematic diagram for explaining the operating principle of the DMD.
As shown in FIG. 29, the projection display apparatus includes a lamp 251 for emitting white light, an ellipsoidal mirror 252 for collecting light emitted from the lamp 251, an UV-IR cut-off filter 253 for eliminating ultraviolet rays and infrared rays from the light emitted from the lamp 251, a rotary color filter 254 that is disposed near a long focus of the ellipsoidal mirror 252 and selectively transmits the three primary colors, red (R), green (G), and blue (B), in sequence, a condensing lens 256, a plane mirror 257, a DMD 258 for modulating incident light to form an optical image, and a projection lens 259 for magnifying and projecting the optical image formed on the DMD 258 onto a screen (not shown).
The rotary color filter 254 is formed by combining red, green, and blue color filters into the form of a disc, and when the rotary color filter 254 is rotated with a motor 255, it can selectively transmit red, green, and blue colors of light of the light collected by the ellipsoidal mirror 252 in sequence. Thus, red, green, and blue colors of illumination light are supplied onto the DMD 258 in sequence.
The condensing lens 256 collects divergent light that has passed though the rotary color filter 254 and directs the light efficiently to the DMD 258 and the projection lens 259.
As shown in FIG. 30, the DMD 258 has a two-dimensional array of microscopic mirrors 261 that are provided in one-to-one correspondence with pixels. For each pixel, tilting of the microscopic mirror 261 is controlled based on the electrostatic field effect of a memory device that is provided directly under the microscopic mirror 261, and the angle of reflection of incident light is changed, thereby forming the ON/OFF states.
Here, a case where the microscopic mirrors tilt±10° with respect to the DMD plane will be described with reference to FIG. 31. FIG. 31 is a schematic diagram showing the operating principle of the microscopic mirrors on the DMD. As shown in FIG. 31, when light 272 tilting 20° with respect to a normal to the device plane of the DVD enters the DMD, if a microscopic mirror 261 is in the ON state (the tilt angle is +10°), then reflected light 273 enters the projection lens 259, and a white pixel is displayed on the screen. On the other hand, if the microscopic mirror 261 is in the OFF state (the tilt angle is −10°), then reflected light 274 does not enter the projection lens 259, and a black pixel is displayed on the screen. Therefore, by controlling the ON/OFF switching for each pixel with time, a gradation expression can be achieved. Moreover, by simultaneously driving the DMD according to the color of illumination light supplied thereto, color display can be performed.
As shown in FIG. 30, the microscopic mirrors 261 on the DMD 258 tilt, for example, in a direction at an azimuth of 45° with respect to a long axis 262 of the display area.
As shown in FIG. 29, the plane mirror 257 is disposed such that it folds the optical path of light from the condensing lens 256 three-dimensionally so as to allow that light to enter the DMD 258 at a predetermined incident angle.
It should be noted that in order to prevent interference between the projection lens 259 and an optical component such as the plane mirror 257, the angle between the optical axis of illumination light and the optical axis of projection light is required to be as large as possible. For this reason, a central axis 258a of the DMD 258 does not coincide with an optical axis 260 of the projection lens 259, and the DMD 258 and the projection lens 259 are arranged with their optical axes offset (shifted) from each other. Therefore, the projection lens 259 uses only a part of the angle of view of an effective image circle to project an optical image formed on the DMD 258.
Generally, projection lenses used in projection display apparatuses as described above are subject to the following requirements.
First, the projection lenses are required to have high resolution. This requirement is important in projecting a high-definition image for high-definition television, for example, and to meet this requirement, it is necessary that the projection lenses have good aberration performance including distortion.
Second, the projection lenses are required to have a low F number. This requirement is important in creating a bright projected image, and to meet this requirement, it is desired that the projection lenses are capable of collecting light from a light valve over a wide angle.
Third, the projection lenses are required to have high aperture efficiency even in the periphery of the screen. This requirement is important in suppressing a decrease in light quantity in a peripheral portion of a projected image on the screen.
Fourth, the projection lenses are required to be capable of realizing projection onto a large screen with a short projection distance. That is to say, it is desired that the projection lenses are wide angle lenses, and to meet this requirement, a lens having a relatively short focal length is necessary.
Fifth, the projection lenses are required to have a sufficiently long back focus space. This requirement is important in separating projection light and illumination light from each other and reserving a sufficient space in which an optical component can be arranged.
Sixth, the projection lenses are required to provide high image quality and high uniformity of brightness.
When actually designing a lens, how to realize these performance requirements rationally in a configuration suitable for mass production is critical.
However, conventional projection lenses and projection display apparatuses using those conventional projection lenses had the following problems.
Generally, to realize a lens with a more rational configuration, reducing the F number and achieving better aberration performance are requirements that are mutually contradictory. Moreover, this applies to increasing the angle of view and reserving a long back focus.
Therefore, it has been very difficult to realize a rational projection lens suitable for mass production while satisfying all of the above-described performance requirements.
Moreover, conventionally, it has been common to arrange the spatial light modulator and the projection lens with their optical axes offset from each other and perform offset projection in order to prevent interference between the optical paths of projection light and illumination light or interference between the projection lens and an optical component such as the plane mirror (see FIG. 29). Offset projection is a method of projecting an image with the display area of the spatial light modulator to be projected being displaced within the effective image circle of the projection lens, and the use of this method causes a loss of the symmetry of the angle of view in the projected image. Consequently, when offset projection was employed, there was a problem in that resolution and brightness of the projected image were asymmetric with respect to the center of the screen. Moreover, only a part of the angle of view of the effective image circle was used, which was wasteful and ran counter to rationalization. Furthermore, when offset projection was employed in a rear projection display apparatus configured with a transmission-type screen, it was necessary to offset the transmission-type screen, too, and thus there was a problem in that offset projection was not suited for rear projection display apparatuses in terms of rationalization.
To address these problems, a configuration (hereinafter, referred to as “right projection”) in which a total reflection prism is disposed between a projection lens and a spatial light modulator (e.g., a DMD) to eliminate the need for offsetting the projection system has been proposed conventionally (see International Publication No. WO98/29773, for example).
However, since the total reflection prism is very expensive, this configuration ran counter to rationalization of a projection system including the projection lens. Moreover, since the total reflection prism contains a minute air layer, there also was a problem of a significant deterioration in the aberration performance of the projection lens due to the gap tolerance of that air layer.